Several 50 feet long single ended runs would be an easy explanation. Some people use similar size antennas to detect space weather and ionosphere effects.
I used to pick up some kind of radio signals with voices that would travel from speaker to speaker. Stereo! They wouldn't last long and I haven't had them in a while.
I always thought the Lawrence Livermore Lab would soak up and attenuate the grid at times. The system seems to sound better late at night, and when the weather is cold (air density?).
There are guys who try to hermetically and electronically seal and standardize their rooms with the usual array of power conditioners and environmental controls. I'm not sure that ultimately works completely except to treat the audiophile's worry warts. However, the audiophile will still have biorhythms and earth frequencies that can vary. Learning to roll with the vagaries of the grid and the body electric as standard cycles of nature might be the more peaceful solution.
Complete control over every variable is probably not possible.